Light emitting diodes(LEDs) are attractive candidates for replacing conventional light sources such as incandescent lamps and fluorescent light sources. The LEDs have higher light conversion efficiencies and longer lifetimes. Unfortunately, LEDs produce light in a relatively narrow spectral band. Hence, to produce a light source having an arbitrary color, a compound light source having multiple LEDs is typically utilized. For example, an LED-based light source that provides an emission that is perceived as matching a particular color can be constructed by combining light from red, blue, and green emitting LEDs. The ratios of the intensities of the various colors sets the color of the light as perceived by a human observer.
Unfortunately, the output of the individual LEDs varies with temperature, drive current, and aging. In addition, the characteristics of the LEDs vary from production lot to production lot in the manufacturing process and are different for different color LEDs. Hence, a light source that provides the desired color under one set of conditions will exhibit a color shift when the conditions change or the device ages. To avoid these shifts, some form of feedback system must be incorporated in the light source to vary the driving conditions of the individual LEDs such that the output spectrum remains at the design value in spite of the variability in the component LEDs used in the light source.
Typically, a prior art light source having a feedback system for maintaining the color perceived by a human observer at a predetermined hue is constructed from a plurality of LEDs that emit light at different wavelengths. A photodetector that includes an appropriate filter is used to measure the light that is generated from each LED. The output of the photodetector is compared to a target value to generate an error signal that is used to adjust the light output of the corresponding LED.
Each target value is a function of the desired light intensity in the corresponding spectral band and of the particular light conversion characteristics of the photodiode that generated the light intensity signal. Hence, even if the circuit designer knows the desired ratio of colors in a light source, the designer must still calibrate the feedback system to take into account the characteristics of the photodiodes. To aid the designer in this regard, prior art systems have been proposed that provide a tricolor light source with inputs that have been pre-calibrated in terms of some standard colorometric system such as the CIE standard.
While such feedback schemes significantly reduce the variability problems discussed above, they require that the circuit designer determine the target values that will generate the desired light spectrum. Consider a circuit designer who is faced with the problem of designing a tricolor LED-based light source to replace a particular incandescent source. The designer must determine the red, green, and blue target values that will provide a spectrum that matches the color of the target light spectrum as perceived by a human observer. If the designer is using a calibrated tricolor light source such as the ones discussed above, the designer must still measure the particular incandescent source in a standard spectrometer to determine the standard values for the target values. Such measurements require specific expertise and increase the cost and product design cycle time.